In spring, many pollen grains swirl in the wind. If one part triggers sneezing and red eyes in allergic individuals, the main part will participate in the sexual reproduction of flowering plants, or angiosperms. Indeed, to reproduce, the flowers release pollen grains intended to join the ovules to fertilize them. Several scenarios arise. In the simplest way, everything happens in the same flower and we speak of “self-fertilization”. In others, corresponding to 10% of angiosperm species, the wind carries the pollen from one flower to another. Finally, in the overwhelming majority of cases, animals participate actively or passively in the transport of pollen which clings to the feathers, fur or even the bristles of insects. This pollination, the process at the origin of the incredible diversity of flowering plants, is not the prerogative of the latter. It has indeed, later, been demonstrated in other groups of terrestrial plants such as mosses and even in the marine environment in aquatic angiosperms. Quid Algae ? In these, it was long assumed that the encounter between male and female gametes depended only on the movement of water. For the first time, the team led by Myriam Valero, CNRS research director at the Roscoff Biological Station, has revealed the role of a small marine crustacean, the idotée (Idotea balthica)in the reproductive success of a red alga, the gracilaire (Gracilaria gracilis). In other words, this is unprecedented proof of marine pollination mediated by an animal in an algae!
The gracilaire is a dioecious algae, that is to say whose male individuals releasing their unflagellated gametes into the water are distinct from the female individuals in which fertilization takes place. Also, the reproductive success of the species depends partly on the distance at which the partners of the opposite sex are, but especially on possible vectors of transport of the male gametes towards the female gametes. However, as this algae likes water with little turbulence, it is unlikely that this transfer is entirely ensured by the currents. But then, how does this algae reproduce?
The male algae are dotted with structures that produce male gametes, the spermatia, surrounded by a sticky substance called “mucilage”. Then comes into play the idotee. When this small crustacean, about twenty millimeters, moves on the male algae, the spermatia adhere to its shell. They will then be deposited on the female algae as the arthropod moves, in order to make fertilization possible.
To verify the role of the crustacean in the reproductive success of the gracilaire, the researchers placed male and female algae in aquariums with or without idotae. The results show that algae reproduce twenty times more efficiently in the presence of crustaceans. The transport of male gametes by idotae was also confirmed by an additional experiment where the researchers were able to observe fertilization after the transfer of idotae placed in an aquarium containing only female algae, the pollinators having been previously “loaded” with spermatia with male algae alone.
In this relationship, the gracilar is not the only winner. Indeed, algae provide shelter and food sources to crustaceans which feed on small organisms growing on the surface of the algae. We speak of “mutualistic interaction” where the benefits are reciprocal for the two actors.
If several aspects of the interaction remain to be studied, in particular to know if the idotees stimulate the release of gametes by the gracilars, this discovery has already had a great impact within the scientific community for other reasons. In particular, this observation suggests that the appearance of pollination by animals, which was thought to date back to 140 million years ago in terrestrial plants, would have appeared much earlier in the marine environment. Nevertheless, this mechanism may also have emerged independently on land and in the oceans. In any case, the history of the plants is to be revised.